Electrical vapor detector



April 1956 P. D. ZEMANY ELECTRICAL VAPOR DETECTOR Filed Aug. 22, 1952 T0 VACUUM SYSTEM 0. C. MICRO/l MMETER Inventor";

Paul D. Zemang,

His Attorney.

United States PatentO 2,742,585 ELECTRICAL 'VAPOR DETECTOR Paul D. Zemany, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application August 22, 1952, Serial No. 305,883 8 Claims. (CL, 313-7) vapors, smokes and similar matter in an atmosphere by directing a sample of an atmosphere suspected of containing a concentration of the substances to be detected into an electrical discharge device under conditions which permit the substance to induce positive ion formation at a more positively charged and heated electrode or anode, V

the positive ions so formed being collected by a negatively charged electrode to produce an indicating current which increases with the concentration of the substance. De-

. vices of this type have been found to be qualitatively selec tive at atmospheric or greater pressures and in vacua; that is, they respond only to certain types of substances such as the alkali metals or other substances having ionization potentials less than the electron work function of the electrodes, the halogens or compounds of either. For certain substances such as the alkali metals and their compounds the ion formation apparently occurs by ionization of the substance when it comes into contact with the 7 more positively charged and heated electrode. For others such as the halogens and their compounds the substance appears to cause ion formation onlyin the presence of what might culled sensitizing materials such as the alkali metals and their compounds, and it further appears in this case that the sensitizing materials themselves are In employing the above-referenced and described device for the detection of halogens and their compounds in vacuayparticularly at pressures of the order of 1 mm. Hg. or lower, I have found that high background currents of positive ions are obtained as a result of the absorption of the halogens'and their compounds by the large bulk of sensitizing material which is utilized. The large bulk of sensitizing material serves as a source of the halogensand their compounds for a considerable length of time following cessation of their admission to the electrical discharge device and the associated vacuum system, whereby high background currents of positive ions result and limit the usefulness of the device. Moreover, I have found that close spacing of the negative and positive electrodes for V the purpose of increasing sensitivity and reducing the difference of potential required to collect the positive ions necessitates elaborate and expensive manufacturing procedures for obtaining such close spacings.

The principal object of my invention is to provide an improved electrical vapor detector of the type described wherein high background currents are absent and expensive manufacturing procedures'are avoided.

According to one important aspect of my invention which is more particularly described hereinafter, I eliminate high background currents and expensive manufacturing procedures in vapor detectors of the type described by coating the positive heated electrode with a refractory material which serves both as an insulator and as a longlife positive ion emitter or sensitizing material at the temperature of operation of the device. In this manner I am able to construct the positive and negative electrodes without taking special precautions to prevent their contact with each other and am additionally able to avoid the use of a large bulk of sensitizing material within the discharge device of the detector.

In the single figure of the drawing, which is a partly schematic simplified representation of an electrical vapor detector according to my invention, there is shown an electric discharge device which comprises a hermetically sealed envelope 1 adapted to be sealed into a vacuum system (not shown) through a tubular connection 2. Supported within envelope 1 by means of rigid conductive leads 3 and 4 is a filament 5 which is coated over its operative portions with a refractory material that will be more fully described hereinafter. Surrounding filament 5 is a tubular'collector electrode 6 of conductive material such as nickel, copper, silver, platinum, etc., which is supported by a rigid conductive lead 7. As shown, the coating on filament 5 can contact electrode 6 at several points. Filament 5 is heated bypassing current therethrough'from asuitable' source 8 of alternating or direct current connected across leads 3 and 4. Interconmeeting leads 3 and 7 is a series circuit which includes a source of direct voltage 9, a current limiting resistor '10 and a direct current microammeter 11. Lead 3 can be connected to ground as illustrated at 12. Electrode 6 must be at a negative potential with respect to filament 5 and therefore, if source 8 is an alternating voltage source, the peak magnitude must be less than the voltage of source 9 by the desired minimum difference of potential between electrode 6 and filament 5.

In the operation of the device of the drawing, the device is evacuated through tubular connection 2, preferably to a pressure of the order of 1 mm. Hg or lower. After evacuation and energization of the device such that filament 5 heats the coating thereon to a temperature ranging from about 700 C. to 12001300 C. or somewhat higher, relatively large ion currents are initially collected upon the negatively charged electrode 6 and indicated upon microammeter 11. When this initial current subsides, the device is prepared to indicate the subsequent admission of the vapors of halogens and their compounds to the vacuum system. Such admission of the vapors of halogens and their compounds causes an increase in the positive ion current collected upon the negatively charged electrode 6. One convenient manner of utilization of the device is to go over the surface of the vacuum system with a probe-directed stream of a gaseous halogen compound such as dichlorodifiuoromethane, whereby leaks in the system can be detected by the increase in the reading of meter 11. 7

One of the most important aspects of my invention concerns the selection of material for coating filament 5 over the portion of its surface which is within electrode 6. This coating according to the invention serves as a source of the alkali metal (e. g. sodium, potassium, etc.) ions or alkali metal compound ions which are collected upon electrode 6 and also as an insulator for preventing electronic conduction between filament 5 and'electrode 6. When the vapors of halogens and their compounds come into contact with the heated coating, the evaporation of alkali ions from the surface of the coating increases thereby increasing the reading of meter 11 and indicating the presence of the halogens. I have found that thin refractory coatings, e. g. coatings of a few mils thickness, of spe ific metal oxides act both -as insulators' and alkali ion emitters at temperatures ranging from about 700 C. to 1-200l300 C. or higher. Moreover, although the quantity of the material of the refractory coating is relatively small in volume, alkali ion emission can be obtained for as long as several hundred hours.

Materials which can be alternatively employed for the refractory coating of filament 5 according to the invention are the oxides of aluminum (alumina), titanium (titania), beryllium (beryllia), thorium (thoria), magnesium (magnesia), calcium, molybdenum, iron, manganese, silicon, cobalt, nickel, and the rare earths (the rare earths have atomic numbers 57 to 71, inclusive, e. g. europium, cerium, gadolinium, etc.). Even in their most purified available forms, these oxides contain at least a few hundrethsv of a per cent of alkali impurities which evaporate as alkali ions during operation of the vapor detector to detect halogens. At the high temperatures of the coating during operation of the device, the oxides of these materials apparently possess semi-conductor properties which allow them to serve as insulators for the direct flow of current between filament 5 and electrode 6 but still permit the evaporation of alkali ions in the presence of halogens. While I do not fully understand the reasons for this dual nature of these materials, I realize that, if these materials remained relatively perfect insulators at high temperatures, continued alkali ion emission could not be obtained because the charge left by removal of a few positive ions from the surface of the coating would efiectivcly prevent further evaporation. Therefore, at the above specified temperatures these materials presumably become sufficiently good semi-conductors to allow migration of charge from the surface of the coating to the filament where the charge is neutralized, but at the same time retain sufiicient insulating properties to prevent electronic conduction between filament 5 and electrode 6 where the coating contacts electrode 6. For reasons of which I am not aware, the oxides of zirconium and hafnium do not serve as successful alkali ion emitters in the device, of'the invention. In any event, the process of. the evaporation of alkali ions from theabove-mentioned materials is fundamentally different from the evaporation of positive ions from the surface of a conductor.

The thickness of the coating uponfilament 5 is, as stated heretofore, preferably of the order of a few mils. The thickness should be as small as possible in order to reduce the bulkof absorbent material within the discharge device but still provide adequate alkali ion emission and insulation. The temperature to which the coating is heated preferably is as high as can be utilized without evaporation of the molecules of the selected material itself, i. e. the material must remain thermally stable. For the; materials of the invention. this temperature ranges from about 700 C. to about 1300 C. The material of filament 5 must, of course, be a conductor capable of withstanding the high temperatures of operation, e. g. tungsten, platinum, etc. The selected material of the coatingcan be deposited upon the filament in conventional fashion foroxide coatcdfilaments by spraying or otherwiselapplying a-colloidal suspension. of granules of the material upon the filament and thereafter baking to eva, oratc theliquid carrier. The filament can have convenient formsor shapes other-.than-the twisted configuration illustrated, in the,- figure, providing. sufiicient passages are provided within electrode 6 t to, permit. exposure of considerahlesurfaee-of, the coating to, vaporsv of halogens or their compounds. introduced into. the vacuum system. While itispreferable for electrodes to be tubular so that a maximum-number of :alkali ions evaporatedfromthe coating can. becollected, semi-tubular or other: convenient shapes can be. utilized.

Apartiallisttof. halogen compounds to :which-the device.

4 is sensitive is as follows. chloroform (CHCla), ferric chloride (FeCls), hydrochloric acid (I-ICl), bromobenzene (CsHsBr), methyl iodide (CHsI), monochlorobcnzene (CsHsCl), methylene chloride (CHzClz), methyl chloride (CHsCl), chlorodifiuoro methane (CHClFz), and trichloroethylene (CHClCClz).

As an example for the purpose of illustration only, I have used a direct voltage of 4.5 volts for source 8 and a direct voltage of 15 volts for source 9. With the device of the invention connected to a vacuum system in which the pressure was about 1X10 mm. Hg, I observed a reading of 1.10 microamperes upon meter 11. On introducing a small pressure of chloroben'ze'ne in the order of 10- mm. Hg the reading of meter 11 increased to 1.60 microamperes.

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications can be made without departing from the invention and I, therefore, aim in the appended claims. to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States:

1. In apparatus for selectively detecting halogen vapors and their compounds, an electrical discharge device comprising an evacuable envelope having a conduit opening thereinto for the introduction of vapors to be detected, positive ion generating means supported within said envelope and consisting of a filament coated with a coating a few mils thick of a refractory material selected from the group consisting of the oxides of aluminum, titanium, beryllium, thorium, magnesium, calcium, molybdenum, iron, manganese, silicon, cobalt, nickel and the rare earths, and which contains at least several hundredths percent of an alkali, said coated filament being the only positive ion emitter present within said envelope, and a positive ion collector electrode supported within said envelope closely adjacent to said coated filament.

2. In a vapor detector wherein alkali ions are selectively formed and collected in the presence of vapors of halogens and their compounds, an electrical discharge device comprising an evacuable envelope having a conduit opening thereinto for the introduction of vapors to be detected, positive ion generating means supported within said envelope and consisting of a filament coated with a coating a few mils of thick of a refractory material which is thermally stable at temperatures between 700 C. and 1300 C. and which contains at least several hundredths percent of an alkali, said coated filament being the only positive ion emitter present within said envelope and having both the property of an alkali ion emitter in the-presence of vapors of halogens and their compounds and the property of an insulator for electronic conduction at temperatures between 700 C. and 1300 C., and a cylindrical positive ion collector electrode supported within said envelope closely surrounding said coated filament.

3. In a vapordete'ctor wherein alkali ions are selectively formed and collected in the presence of vapors of halogensand their compounds an electrical discharge device. comprising an evacuable envclops having a conduit opening thereinto for the introduction of vapors to be detected, positive ion generating means supported within said envelope and consisting of a filament coated with a coating a few mils thick of a refractory material selected from the group consisting of the oxides of aluminum, titanium, beryllium, thorium, magnesium, calcium, molybdenum, iron, manganese, silicon, cobalt, nickel and the rare earths, and containingat least several hundredths percent of an alkali, said coated filament being the only positive ion emitter present within said envelope and having both the. property of' an alkali ionemitter in the presence of vaporsiof halogens'and their compounds and the property of an insulator for electronic conduction at temperatures between 700 C. and 1300 C., an'd apositive ion collector electrode supported within said envelope closely adjacent to said coated filament. v

4. The device of claim 3 in which the refractory coating upon the filament is alumina.

5. The device of claim 3 in which the refractory coating upon the filament is titania.

6. The device of claim 3 in which the refractory coating upon the filament is beryllia.

7. The device of claim 3 in which the refractory coating upon the filament is thoria.

8. The device of claim 3 in which the refractory coating upon the filament is magnesia.

References Cited in the file of this patent UNITED STATES PATENTS 

1. IN APPARATUS FOR SELECTIVELY DETECTING HALOGEN VAPORS AND THEIR COMPOUNDS, AN ELECTRICAL DISCHARGE DEVICE COMPRISING AN EVACUABLE ENVELOPE HAVING A CONDUIT OPENING THEREINTO FOR THE INTRODUCTION OF VAPOR TO BE DETECTED, POSITIVE ION GENERATING MEANS SUPPORTED WITHIN SAID ENVELOPE AND CONSISTING OF A FILAMENT COATED WITH A COATING A FEW MILS THICK OF A REFRACTORY MATERIAL SELECTED FROM THE GROUP CONSISTING OF THE OXIDES OF ALUMINUM, TITANIUM, BERYLLIUM, THORIUM, MAGNESIUM, CALCIUM, MOLYBDENUM, IRON, MANGANESE, SILICON, COBALT, NICKEL AND THE RARE EARTHS, AND WHICH CONTAINS AT LEAST SEVERAL HUNDREDTHS PERCENT OF AN ALKALI, SAID COATED FILAMENT BEING THE ONLY POSITIVE ION EMITTER PRESENT WITHIN SAID ENVELOPE, AND A POSITIVE ION COLLECTOR ELECTRODE SUPPORTED WITHIN SAID ENVELOPE CLOSELY ADJACENT TO SAID COATED FILAMENT. 